A light emitting diode (LED), which is widely used as a light source of an electronic device in recent years, is characterized in its being low power consumption, having a long service life, and being compact in size. A current driver circuit supplies a forward current that is required to light up such a light emitting element.
In such a conventional current driver circuit, as shown in FIG. 7, a DC-DC converter 2 which converts an input power supply 1 into a DC voltage is used. By connecting the output of the DC-DC converter 2 to a series circuit of LEDs 3 and a current detecting resistor 40, the DC-DC converter 2 is controlled such that a voltage drop at the current detecting resistor 40 caused by the current flowing through the LEDs 3 is stabilized at a prescribed value (see Patent Document 1). In the conventional current driver circuit shown in FIG. 7, the DC-DC converter 2 is structured as a boost converter assuming that an Li-ion battery of about 3.5 V is used as the input power supply 1, and that the LEDs 3 of forward voltage of several V are cascaded to the output.
The DC-DC converter 2 being a boost converter shown in FIG. 7 is structured with: a coil 20 connected to the input power supply 1; a switching element 21 and a diode 22 connected to the other end of the coil 20; an output capacitor 23 connected to a cathode of the diode 22; and a control circuit 24 that controls the drive operation of the switching element 21 to stabilize a feedback voltage Vfb being the voltage of the current detecting resistor 40 at a prescribed value. When the switching element 21 is in an ON state, an input voltage Vi is applied to the coil 20, and a current flows through the coil 20 and energy is stored. Next, when the switching element 21 enters an OFF state, a current that charges the output capacitor 23 flows from the input power supply 1 through via the coil 20 and the diode 22, and the energy stored in the coil 20 is released. By the foregoing operations repeatedly performed, an output voltage Vo is generated in the output capacitor 23, to supply the LEDs 3 with a current Iled.
Assuming that the duty ratio (the proportion of an ON time period in a switching period) of the switching element 21 of the DC-DC converter 2 being a boost converter is D, and ignoring any voltage drop such as in the forward voltage of the diode 22, the relationship between input and output voltages can be represented by the following equation (1):
                    Vo        =                  Vi                      1            -            D                                              (        1        )            
The control circuit 24 receives, as a feedback voltage Vfb, a voltage drop in the current detecting resistor 40 which is caused by the supply current Iled flowing into the LEDs 3, and the control circuit 24 drives ON/OFF the switching element 21 while adjusting the duty ratio D of the switching element 21, such that the feedback voltage Vfb attains a prescribed value. By the foregoing operations, the supply current Iled to the LEDs 3 is controlled to be stabilized.